WASHINGTON -- In a victory for Ford Motor Co., federal regulators have decided not to formally investigate claims that thousands of Ford Explorers built between 1997 and 2001 may have unsafe roofs that violate safety standards.

The National Highway Traffic Safety Administration last week rejected a petition by California-based Safety Analysis & Forensic Engineering Research to retest 10 vehicles built during the period.

NHTSA denied the request after a year of reviewing a SAFE Research contention that thousands of Explorers likely didn't meet Federal Safety Standard 216, which requires that vehicle roofs withstand a force equal to at least 1.5 times the vehicle's unloaded weight.

The group, which does work for lawyers suing automakers, based its findings on two roof strength tests Ford conducted in 1999, which the company said were to test a prototype windshield design that was never put into production.

SAFE Research contends that the tests -- on a minor proposed design change -- showed that many Explorers built during that time period may have failed. Ford redesigned the Explorer and the Mountaineer in 2002.

"The windshield modifications that Ford was considering when it modified and then tested these vehicles in 1999 never became part of production vehicles," wrote Daniel C. Smith, NHTSA's associate administrator for enforcement.

SAFE had asked NHTSA to test Explorers in question to determine whether they met the roof standard, but the agency said it didn't have the authority to test used vehicles. "We believe NHTSA's limited resources are better allocated to investigations that are more likely to reveal noncompliance," Smith wrote.

Ford spokesman Dan Jarvis praised NHTSA's decision.

"We felt all along and still do that this report SAFE issued a year ago was based on junk science," Jarvis said Friday. "You can't take a prototype vehicle and make the leap somehow that it relates to vehicles on the road."

In recent years, Ford has faced hundreds of lawsuits stemming from Explorer rollovers. And the company has been hit with several multi-million dollar jury verdicts in cases in which plaintiffs alleged the Explorer's roof was too weak.

Rollover deaths continue to rise in the United States, accounting for 25 percent of all traffic deaths, up 2.1 percent to 10,816 in 2005, according to a government report NHSTA issued last month.

The documents cited by SAFE Research were introduced as evidence in a trial in Texas in 2005 stemming from an Explorer rollover accident that killed two teenagers. The case was settled before a verdict was issued.The NHTSA decision comes as the agency continues to review the question of roof strength.